1. Field of the Invention
The present invention relates to a piezoelectric vibrator and a manufacturing method thereof. More particularly, the present invention relates to a piezoelectric vibrator which utilizes a vibration mode such as an expansion mode, bending mode or contour mode and a manufacturing method thereof.
2. Description of Prior Art
FIG. 1 is a perspective view showing an example of a conventional piezoelectric vibrator being the background of the present invention. A piezoelectric vibrator 10 includes a substrate 12 made of Elinvar, for instance, and the substrate 12 includes a tetragonal frame and an island-like vibrating element 14 surrounded thereby. The island-like vibrating element 14 is connected and supported with two opposite sides of the frame by connecting portions 16, 16. On the top surface of the vibrating element 14 there is formed a piezoelectric film 18 made of, for instance, zinc oxide (ZnO) by e.g. sputtering. The piezoelectric film 18 is deposited to extend to a portion of the frame of the substrate 12 through the connecting portion 16. On the surface of the piezoelectric film 18 formed on the vibrating element 14 there is formed an electrode 20 made of aluminum or the like. The electrode 20 is extended to a lead electrode 22 through the connecting portion 16. External terminals are connected to the lead electrode 22. The vibrating element 14 on which the piezoelectric film 18 is deposited undergoes vibration in e.g. expansion mode when an electric field is applied between a proper position of the frame of the substrate 12 and the lead electrode 22.
If the vibration mode is expansion mode, the resonant frequency of the piezoelectric vibrator shown in FIG. 1 is determined by the formula f=K(1/L), wherein K is a constant, and L is the length of the vibrating element 14 in the section taken along the line II--II in FIG. 1. Thus, the length L is one of the important factors determining the vibrating or resonant frequency of a given piezoelectric vibrator.
On the other hand, the method of wet etching has hitherto been used for forming such piezoelectric vibrators. Since etching proceeds in the longitudinal direction (along the surface) in the same speed as it progresses in the direction of thickness, consequently the dimensional precision of the work depends on the thickness T (FIG. 2). That is, when etching is used, the end portion of the vibrating element is eliminated in the longitudinal direction to the same extent as thickness T, this aggravating the dimensional stability. Hitherto, to improve the dimensional precision attempt has been made to do etching from both sides, but even this method is seen causing projection about the center of the thickness T as shown in FIG. 2, this interfering with sufficient dimensional precision. More specifically, even in the conventional both-side etching, dimensional scattering of some 1/10 of the thickness T has been inevitable, this being reflected in scattering of resonant frequency of the piezoelectric vibrator. In order to suppress the scattering of resonant frequency it is necessary to improve the dimensional precision and in order to improve the dimensional precision it is necessary to reduce the thickness T. However, the thickness cannot be reduced drastically considering its influence on the plate's mechanical strength, change of temperature property caused by altered structural ratio with respect to the piezoelectric layer, and the warping stress being subjected to during sputtering of the piezoelectric film. Thus, the limit of dimensional precision caused by thickness of the substrate or vibrating element presents a great obstacle for further miniaturization of piezoelectric vibrator.